This invention relates to an improved process for the construction of trenches in ground containing vapor and/or water. More particularly, the invention relates to the use of a certain polymer material as the support media introduced during the trenching to support the trench walls formed.
The ability to excavate the earth in the form of narrow trenches with the sole support of a liquid slurry akin to a drilling mud is now common practice for different kinds of constructions such as reinforced diaphragm walls, soldier beam tremie concrete walls, soil bentonitc slurry trenches of self-hardening slurry trenches, either for structural or for ground water control applications, respectively. A more recent development is the use of natural and synthetic polymer slurries in the trenching of deep collection drains. Although the first application was for draining ground water causing slope instability, the present applications are confined to the extraction of contaminated groundwater or decomposition gases such as methane. In situ bioremediation can also use polymer slurry trenched drains for extraction and reinjection of treatment fluids. A new application consists in combining conventional slurry trenched barrier with polymer slurry trenched drain segments filled with reactive particulates to channel contaminated ground water through these "gates" and provide in situ treatment "on the run".
Typically, trenching under slurry causes some soil particles to become suspended. In traditional thixotropic clay slurries, most of the fines remain suspended when the slurry comes at rest. On the contrary, for almost all polymer slurries used in the trenching process, fines are only temporarily in suspension while the slurry is agitated but, when at rest, all fines will ultimately settle. This creates two types of problems in the construction of polymer slurry trenched drains. The suspended fines will combine with the polymer in forming a filter cake on the pervious sides of the trench. Theses fines will have to be removed from the soil/filter sand/gravel backfill interface to reestablish the natural conductivity of the ground. The fines that will have to settle into a sludge at the bottom of the trench within the lapse of time between trenching and backfilling need to be removed to permit the placement of a drain pipe and of the granular backfill in clean conditions. If not, the granular backfill filter comes, in this critical lower stage, saturated with the sludge. Once installation is complete, both situations require a serious cleaning effort during the drain development which is quite similar to that of a water well. The fact that these techniques are done "in the blind" is conducive to difficult control and open to poor practices leading to mediocre performance. This result is often masked by the fact that the extraction flow rate required by the treatment plant is often a fraction of the drains hydraulic yield potential.
Since trenching processes using polymer slurry as the support media for the walls formed by the trenching carries serious drawbacks as described above, there is a need for the materials other than polymer slurries that provide proper support and performances.